1. Field of the Invention
The present invention relates to a picture quality improvement device of video signals and a picture quality improvement method of video signals.
2. Description of the Related Art
In an image display device such as a television image receiver or video projector, picture quality is known to deteriorate due to the occurrence of flare. Flare is a phenomenon in which the reflection or scattering of light on the irradiation surface or lens of a picture tube or projector tube causes the intrusion of light of a bright area into a dark area, thereby producing blurring of edges at which the differences in the luminance in a displayed image are great (for example, at the borders of white regions and black regions).
To correct this type of flare, image processing is carried out to emphasize edges having large differences in luminance in a displayed image. Referring to FIG. 1, a block diagram is shown that shows an example of the configuration of a picture quality improvement device of the prior art for correcting flare by image processing for emphasizing edges (Refer to JP-A-H01-246984 (Patent Document 1) or JP-A-H01-246985 (Patent Document 2)). In FIG. 1, image processing is carried out to emphasize edges for the Y signal of a luminance (Y)/color difference (C) signal.
In FIG. 1, Y input signal (Yin) is applied as input to delay compensation circuit 31 and two-dimensional low-pass filter (LPF) circuit 32. Delay compensation circuit 31 is a circuit for delaying the input signal the time required for the processing of two-dimensional LPF circuit 32. Two-dimensional LPF circuit 32 is a filter for eliminating from the input signal frequency components (such as edge components) that have higher frequency than a prescribed frequency. Two-dimensional LPF circuit 32 is made up from, for example, a delay circuit, an amplification circuit, and an addition circuit; and eliminates the high-frequency component of the input signal by replacing the data of a particular picture element with, for example, the weighted average of data of a plurality of adjacent picture elements (refer to Patent Document 2).
Due to the elimination of the high-frequency component of the Y input signal that is applied as input to two-dimensional LPF circuit 32, a signal having dulled edges is supplied as output from two-dimensional LPF circuit 32 (refer to the waveform shown in FIG. 1). The Y input signal that has been delayed by delay compensation circuit 31 a time interval that corresponds to the processing time of two-dimensional LPF circuit 32 and the signal having dulled edges that is supplied as output from two-dimensional LPF circuit 32 are applied as input to subtraction circuit 34. Subtraction circuit 34 supplies as output a signal in which the latter signal has been subtracted from the former signal. Accordingly, subtraction circuit 34 supplies as output a signal in which the high-frequency component (edge component) that was eliminated by two-dimensional LPF circuit 32 has been extracted (Refer to the waveform shown in FIG. 1). Amplification circuit 35 multiplies the signal in which the high-frequency component has been extracted and that has been supplied as output from subtraction circuit 34 by a prescribed factor and supplies this signal to addition circuit 36 (Refer to the waveform that is shown in FIG. 1). Addition circuit 36 adds the signal that was supplied as output from amplification circuit 35, in which the high-frequency component was extracted and that was then multiplied by a prescribed factor, to the Y input signal that is supplied from delay compensation circuit 31. The resulting Y output signal (Yout) is a signal in which the edge components of the Y input signal have been emphasized (Refer to the waveform that is shown in FIG. 1). The above-described process thus realizes flare correction. In addition, the C input signal (Cin) is subjected to a delay process by delay compensation circuit 33 for the time interval of the processing of two-dimensional LPF circuit 32, subtraction circuit 34, amplification circuit 35, and addition circuit 36, and is then supplied as a C output signal (Cout).
In the foregoing explanation, flare correction was carried out only for the Y signal because, of the luminance/color difference signal, flare correction in the Y signal has the greatest effect on picture quality improvement. Obviously, flare correction may also be carried out not only for the Y signal but for the C signal as well. Flare correction may also be carried out for the RGB (Red, Green, and Blue) signals (in which case, flare correction for the G signal has the greatest effect on picture quality improvement).
According to, for example, the standards of the NTSC (National Television System Committee), the luminance/color difference signal is often transmitted as an interlaced signal (RGB signals are not often transmitted as interlaced signals). Referring to FIG. 2, a schematic view is shown for explaining an interlaced signal in comparison with a progressive signal (a), which is the counter-concept of an interlaced signal (b). In an interlaced signal, in contrast to a progressive signal, a video signal that corresponds to odd-numbered rows in the horizontal direction (y direction) in FIG. 2 is first transmitted, following which a video signal that corresponds to even-numbered rows is transmitted. The video signal (F1) that corresponds to odd-numbered rows and the video signal (F2) that corresponds to even-numbered rows are referred to as “field signals,” and the two field signals make up one frame. In the following explanation, the data string that makes up F1 is s1, s3, s3, . . . , and the data string that makes up F2 is t1, t2, t3, . . . .
Using the picture quality improvement device of FIG. 1 to correct flare in a luminance/color difference signal that is transmitted as this type of interlaced signal gives rise to the problem described hereinbelow. Referring to FIG. 3A and 3B, schematic views are shown for explaining this problem. In addition, FIG. 3B is a schematic view for explaining this problem based on sections that are taken along line “a” in each view of FIG. 3A.
When flare correction is carried out for a luminance/color difference signal that is transmitted as an interlaced signal, flare is normally corrected separately for video signal (F1) that corresponds to odd-numbered rows and video signal (F2) that corresponds to even-numbered rows in the picture quality improvement device of FIG. 1, following which these video signals are combined.
When this method is used, however, flare correction cannot be realized accurately when the video image changes with each row. For example, if luminance exists only in odd-numbered rows (F1) as shown by input signal (a) in FIG. 3A and FIG. 3B, resolving the input signal of FIG. 3A and FIG. 3B to the F1 signal and F2 signal results in the F1 signal (b) that is identical to the input signal and F2 signal (c) that simply represents the background signal. Separate implementation of flare correction for these field signals will then result in emphasized edges for the F1 signal (d), but for the F2 signal, which is merely the background signal, edges will not be emphasized (e). Subsequent combination of these images results in the generation of an unnatural picture (f) in which no edge emphasis occurs in even-numbered rows. For the purpose of reference, a picture that has undergone appropriate flare correction is also shown (g).
To avoid this problem, a solution can be considered in which interlaced/progressive conversion circuit 61 is provided before picture quality improvement device 62, as shown in FIG. 4, whereby flare correction is carried out by the picture quality improvement device of FIG. 1 (picture quality improvement device 62) after first converting the interlaced signal to a progressive signal. Adoption of this approach solves the above-described problem because flare correction is not carried out separately for the F1 signal and F2 signal. However, the sampling clock frequency of the progressive signal following conversion is twice the sampling clock frequency of the interlaced signal before conversion, and this increase in frequency raises the problem that two-dimensional LPF circuit 32 is required to perform high-speed processing, and this high-speed processing imposes an excessive load on the circuit.